High-efficiency diffraction gratings are often useful in laser systems that employ very-high-power laser beams. In particular, systems that use spectral-beam combining to increase the total power of a single collimated laser beam to power levels of one megawatt or more have a need for high-efficiency (low-loss) diffraction gratings.
U.S. Pat. No. 7,199,924 to Brown et al. issued Apr. 3, 2007, titled “Apparatus and method for spectral-beam combining of high-power fiber lasers,” is assigned to the owner of the present application, and is incorporated herein by reference. In U.S. Pat. No. 7,199,924 Brown et al. describe an apparatus and method for spectral-beam combining light from a plurality of high-power fiber lasers that, in some embodiments, use two substantially identical diffraction gratings in a parallel, mutually compensating configuration to combine a plurality of separate parallel input beams each having a slightly different successively higher wavelength into a single output beam of high quality. In other embodiments, a single diffraction grating is used to combine a plurality of different wavelengths, wherein the input laser beams are obtained from very narrow linewidth sources to reduce chromatic dispersion. In some embodiments, diagnostics and adjustments of wavelengths and/or positions and angles are made dynamically in real time to maintain the combination of the plurality input beams into a single high-quality output beam.
U.S. Pat. No. 8,526,110 to Honea et al. issued Sep. 3, 2013, titled “Spectral-beam combining for high-power fiber-ring-laser systems,” is assigned to the owner of the present application, and is incorporated herein by reference. In U.S. Pat. No. 8,526,110 Honea et al. describe a ring-laser system that includes a plurality of ring-laser gain elements and a spectral-beam-combining output stage configured to combine a plurality of beams coming from the gain elements into an output beam and that includes chromatic-dispersion compensation. In some embodiments, the output stage includes a plurality of highly reflective dielectric-coated focussing elements. In some embodiments, the output stage includes a plurality of high-efficiency dielectric-coated grating elements. In some embodiments, the output stage includes a mostly reflective but partially transmissive output mirror and a highly reflective beam-reversing mirror configured to reflect a majority of a backward-traveling signal beam such that it becomes forward traveling. In some embodiments, each gain element further includes a photonic-crystal-rod power amplifier. Some embodiments have an amplitude modulator configured to pulse the plurality of beams, and a timing controller configured to synchronize the pulses of the plurality of beams. Some embodiments further include a non-linear wavelength-conversion device.
U.S. Pat. No. 8,503,840 to Hu et al. issued Aug. 6, 2013 titled “Optical-fiber array method and apparatus,” is assigned to the owner of the present application, and is incorporated herein by reference. In U.S. Pat. No. 8,503,840, Hu et al. describe a method and apparatus for forming an optical-fiber-array assembly, which includes providing a plurality of optical fibers including a first optical fiber and a second optical fiber, providing a fiber-array plate that includes a first surface and a second surface, connecting the plurality of optical fibers to the first surface of the fiber-array plate, transmitting a plurality of optical signals through the optical fibers into the fiber-array plate at the first surface of the fiber-array plate, and emitting from the second surface of the fiber-array plate a composite output beam having light from the plurality of optical signals. Optionally, the first surface of the fiber-array plate includes indicia configured to assist in the alignment of the plurality of optical fibers on the first surface of the fiber-array plate. In some embodiments, the second surface of the fiber-array plate includes a plurality of beam-shaping optics configured to shape the composite output beam.
U.S. Pat. No. 8,493,651 to Hu et al. issued Jul. 23, 2013 titled “Apparatus for optical fiber management and cooling,” is assigned to the owner of the present application, and is incorporated herein by reference. In U.S. Pat. No. 8,493,651, Hu et al. describe an apparatus and method that provides management and cooling of an optical fiber by looping the optical fiber around the inner surface of a heat-conductive cylinder and around the outer surface of the heat-conductive cylinder, such that the optical fiber enters and exits the heat-conductive cylinder on substantially the same plane. Some embodiments use a continuous groove on the inside and outside of the cylinder for guiding and managing the optical fiber. Some embodiments use a plurality of protruding fiber guides for guiding and managing the optical fiber. Some embodiments use an integrated tube for guiding and managing the optical fiber. In some embodiments, the optical fiber looped on the inner surface and outer surface are spaced apart substantially equally. In some other embodiments, the optical fiber loops are spaced further apart for portions of the fiber carrying higher power.
U.S. Pat. No. 8,441,718 to Mead issued May 14, 2013 titled “Spectrally beam combined laser system and method at eye-safer wavelengths,” is assigned to the owner of the present application, and is incorporated herein by reference. In U.S. Pat. No. 8,441,718, Mead describes a method and system in which fiber-laser light is Raman shifted to eye-safer wavelengths prior to spectral beam combination, enabling a high-power, eye-safer wavelength directed-energy (DE) system. The output of Ytterbium fiber lasers is not used directly for spectral beam combining. Rather, the power from the Yb fiber lasers is Raman-shifted to longer wavelengths, and these wavelengths are then spectrally beam combined. Raman shifting is most readily accomplished with a “cascaded Raman converter,” in which a series of nested fiber cavities is formed using fiber Bragg gratings.
U.S. Pat. No. 8,411,712 to Honea, et al. issued Apr. 2, 2013 titled “Beam diagnostics and feedback system and method for spectrally beam-combined lasers,” is assigned to the owner of the present application, and is incorporated herein by reference. In U.S. Pat. No. 8,411,712, Honea, et al. describe an apparatus and method for control of lasers (which use an array of optical gain fibers) in order to improve spectrally beam-combined (SBC) laser beam quality along the plane of the SBC fiber array via spectral-to-spatial mapping of a portion of the spectrally beam-combined laser beams, detection of optical power in each of the spatially dispersed beams and feedback control of the lasers for wavelength-drift correction. The apparatus includes a diffractive element; a source of a plurality of substantially monochromatic light beams directed from different angles to a single location on the diffractive element, wherein the diffractive element spectrally combines the plurality of light beams into a single beam. A controller adjusts characteristics of the light beams if one of the light beams has become misadjusted. In some embodiments, the controller adjusts the wavelength tuning of the respective fiber laser.
U.S. Pat. No. 8,199,399 to Savage-Leuchs issued Jun. 12, 2012 titled “Optical gain fiber having segments of differing core sizes and associated method,” is assigned to the owner of the present application, and is incorporated herein by reference. In U.S. Pat. No. 8,199,399, Savage-Leuchs describes an apparatus and method for amplifying laser signals using segments of fibers of differing core diameters and/or differing cladding diameters to suppress amplified spontaneous emission and non-linear effects such as four-wave mixing (FWM), self-phase modulation, and stimulated Brillouin and/or Raman scattering (SBS/SRS). In some embodiments, different core sizes have different sideband spacings (spacing between the desired signal and wavelength-shifted lobes). Changing core sizes and providing phase mismatches prevent buildup of non-linear effects. Some embodiments further include a bandpass filter to remove signal other than the desired signal wavelength and/or a time gate to remove signal at times other than during the desired signal pulse. Some embodiments include photonic-crystal structures to define the core for the signal and/or the inner cladding for the pump. Some embodiments include an inner glass cladding to confine the signal in the core and an outer glass cladding to confine pump light in the inner cladding.
U.S. Pat. No. 8,179,594 to Tidwell, et al. issued May 15, 2012 titled “Method and apparatus for spectral-beam combining of fanned-in laser beams with chromatic-dispersion compensation using a plurality of diffractive gratings,” is assigned to the owner of the present application, and is incorporated herein by reference. In U.S. Pat. No. 8,179,594, Tidwell, et al. describe an apparatus and method for spectral-beam combining of light from a plurality of high-power lasers (e.g., fiber MOPA lasers) that, in some embodiments, use substantially identical diffraction gratings in a 1-D non-parallel, mutually compensating configuration to combine non-parallel converging input beams in one plane each having a slightly different successively higher wavelength into a single output beam of high quality. In other embodiments, an output grating and one or more input gratings in a 1-D parallel, mutually compensating configuration combine non-parallel input beams in one plane into a single output beam of high quality. In other embodiments, a 2-D plurality of input gratings in a non-parallel configuration combine a plurality of non-parallel input beams not in one plane each having a slightly different successively higher wavelength into a set of converging beams in one plane directed towards an output grating that compensates for chromatic dispersions introduced by the input gratings.
U.S. Pat. No. 7,065,107 to Hamilton, et al. issued Jun. 20, 2006 titled “Spectral beam combination of broad-stripe laser diodes,” is assigned to the owner of the present application, and is incorporated herein by reference. In U.S. Pat. No. 7,065,107, Hamilton, et al. describe a method and apparatus for improving the beam quality of the emissions from a multimode gain medium such as a broad-stripe laser through the use of SBC techniques is provided. In order to achieve the desired beam quality without a significant reduction in output power, discrete lasing regions are formed across the gain medium using an etalon or similar device located within the SBC cavity.
U.S. Pat. No. 8,094,689 to Koplow issued Jan. 10, 2012 titled “Laser systems configured to output a spectrally-consolidated laser beam and related methods,” and is incorporated herein by reference. In U.S. Pat. No. 8,094,689, Koplow describes a laser apparatus that includes a plurality of pumps each of which is configured to emit a corresponding pump laser beam having a unique peak wavelength. The laser apparatus includes a spectral beam combiner configured to combine the corresponding pump laser beams into a substantially spatially-coherent pump laser beam having a pump spectrum that includes the unique peak wavelengths, and first and second selectively reflective elements spaced from each other to define a lasing cavity including a lasing medium therein. The lasing medium generates a plurality of gain spectra responsive to absorbing the pump laser beam. Each gain spectrum corresponds to a respective one of the unique peak wavelengths of the substantially spatially-coherent pump laser beam and partially overlaps with all other ones of the gain spectra. The reflective elements are configured to promote emission of a laser beam from the lasing medium with a peak wavelength common to each gain spectrum.
U.S. Pat. No. 6,754,006 titled “Hybrid metallic-dielectric grating” issued Jun. 22, 2004 to Barton et al. and is incorporated herein by reference. This patent describes a diffraction grating having a metallic base layer and layers of dielectric materials of varying refractive index, where a bottom interface of the layers is adherent to the metallic base layer. The dielectric layers are periodically spaced on top of the metallic base layer, leaving the metallic base layer exposed in regions. This grating allows for the polarization-insensitive reflective properties of the base metallic layer to operate in conjunction with the polarization sensitive diffraction properties of the multilayer grating structure to provide near 100% diffraction efficiency over a reasonable wavelength bandwidth, independent of the polarization of the incident beam.
U.S. Pat. No. 6,822,796 to Takada et al. titled “Diffractive optical element” (incorporated herein by reference) describes a method for making blazed gratings having asymmetric grooves with dielectric coatings. U.S. Pat. No. 6,958,859 to Hoose et al. titled “Grating device with high diffraction efficiency” (incorporated herein by reference) describes a method for making gratings having dielectric coatings.
U.S. Pat. No. 5,907,436 titled “Multilayer dielectric diffraction gratings” issued May 25, 1999 to Perry et al., and is incorporated herein by reference. This patent describes the design and fabrication of dielectric grating structures with high diffraction efficiency. The gratings have a multilayer structure of alternating index dielectric materials, with a grating structure on top of the multilayer, and obtain a diffraction grating of adjustable efficiency, and variable optical bandwidth.
PCT Publication No. WO 2013/062584, which published May 2, 2013, of PCT Patent Application PCT/US11/58352 titled “Devices including a diamond layer” filed Oct. 28, 2011 by Liang et al., is incorporated herein by reference. Liang et al. describe a device that includes a substrate layer, a diamond layer, and a device layer. The device layer is patterned. The diamond layer is to conform to a pattern associated with the device layer.
U.S. Pat. No. 6,830,813 to Ravi, which issued Dec. 14, 2004 and is titled “Stress-reducing structure for electronic devices,” is incorporated herein by reference. Ravi describes an electronic apparatus having a heat transfer/stress-reducing layer combined with a device layer and methods of fabricating such electronic apparatus provide a means for incorporating a heat transfer layer in an integrated circuit. A structure with a diamond layer incorporated beneath a device layer provides a heat transfer layer for the structure. In an embodiment, a compliant layer is formed between a diamond layer and a substrate to provide stress reduction. In another embodiment, a diamond layer is formed as a layer of islands of diamond from nucleation centers to provide stress reduction.
U.S. Pat. No. 7,501,330 to Ravi, et al., which issued Mar. 10, 2009 and is titled “Methods of forming a high conductivity diamond film and structures formed thereby,” is incorporated herein by reference. Ravi, et al. describe a method of forming a high thermal conductivity diamond film and its associated structures comprising selectively nucleating a region of a substrate, and forming a diamond film on the substrate such that the diamond film has large grains, which are at least about 20 microns in size. The larger grained diamond film has greatly improved thermal management capabilities and improves the efficiency and speed of a microelectronic device.
United States Patent Application Publication 2006/0113545 of Weber et al. was published Jun. 1, 2006, titled “Wide bandgap semiconductor layers on SOD structures,” and is incorporated herein by reference. Weber et al. describe multi-layered structures containing GaN on SOD (silicon/diamond/silicon) substrates. The substrate/epilayer combination can provide electronic materials suitable for high-power and opto-electronic devices without commonly observed limitations due to excess heat during device operation. The resulting devices have built-in thermal heat spreading capability that result in better performance and higher reliability.
U.S. Pat. No. 8,236,594 issued to Sung on Aug. 7, 2012, titled “Semiconductor-on-diamond devices and associated methods,” and is incorporated herein by reference. U.S. Pat. No. 8,236,594 describes semiconductor-on-diamond devices and methods for making such devices. One such method may include depositing a semiconductor layer on a semiconductor substrate, depositing an adynamic diamond layer on the semiconductor layer opposite the semiconductor substrate, and coupling a support substrate to the adynamic diamond layer opposite the semiconductor layer to support the adynamic layer.
U.S. Pat. No. 7,846,767 to Sung issued Dec. 7, 2010 titled “Semiconductor-on-diamond devices and associated methods,” and is incorporated herein by reference. U.S. Pat. No. 7,846,767 describes semiconductor-on-diamond (SOD) substrates and methods for making such substrates. In one aspect, a method of making an SOD device is provided that includes etching depressions into an etch surface of a semiconductor substrate to a uniform depth, depositing a diamond layer onto the etch surface to form diamond-filled depressions, and thinning the semiconductor substrate at a thinning surface opposite the etch surface until the diamond filled depressions are exposed, thus forming a semiconductor device having a thickness substantially equal to the uniform depth.
Even with high-efficiency multi-layered dielectric diffraction gratings such as those described above, a non-negligible amount of energy is absorbed in the grating, which heats and distorts the grating.
There remains a need in the art for improved gratings that improve heat extraction, minimize grating distortion due to unequal coefficients of thermal expansion (CTE), and improve efficiency to reduce grating heating.